1. Field of the Invention
The invention relates to a rotating shaft support apparatus which supports rotating shafts via a structure in which they can rotate relative to one another, such as a differential gear unit that transmits torque to left and right output shafts supported via a structure in which they can rotate relative to one another. More specifically, the invention relates to a lubricant groove for supplying lubricant between a case and rotating shafts supported via a structure that allows them to rotate relative to one other in the case.
2. Description of the Related Art
As an example of a rotating shaft support apparatus that has a case which supports rotating shafts via a structure in which they are able to rotate relative to one another, a differential gear unit is known that transmits rotational driving force, and distributes torque, of a differential case to left and right side gears arranged about a rotational axis of the differential case via pinion gears housed in the differential case. The differential case of this kind of differential gear unit has insertion holes into which output shafts are fit and inside which lubrication grooves for supplying lubricant are often provided.
For example, FIG. 1 of JP(A) 8-170718 shows a spiral-shaped lubrication groove. Forming the lubrication groove in a spiral is advantageous in that it enables the lubricant to travel in the axial direction by the rotational force of the output shafts. Because of this advantage, spiral-shaped lubrication grooves are frequently employed.
The spiral-shaped lubrication grooves, however, are unable to be made during casting of the differential case. This is because even if a core having a spiral projection corresponding to the spiral groove were to be molded, the lubrication groove formed in the die would interfere with the spiral projection formed on the core when the core is moved in a direction perpendicular to the mold to extract it therefrom, resulting in undercutting of the spiral projection. Therefore, the spiral lubrication groove must be machined out after the casting process, which increases man-hours and thus manufacturing costs.